In electrophotographic and electrostatic printing processes (collectively electrographic processes), an electrostatic image is formed on the surface of a photoreceptive element or dielectric element, respectively. The photoreceptive element or dielectric element can be an intermediate transfer drum or belt or the substrate for the final toned image itself, as described by Schmidt, S. P. and Larson, J. R. in Handbook of Imaging Materials Diamond, A. S., Ed: Marcel Dekker: New York; Chapter 6, pp 227-252, and U.S. Pat. Nos. 4,728,983, 4,321,404, and 4,268,598.
Electrophotography forms the technical basis for various well-known imaging processes, including photocopying and some forms of laser printing. Other imaging processes use electrostatic or ionographic printing. Electrostatic printing is printing where a dielectric receptor or substrate is “written” upon imagewise by a charged stylus, leaving a latent electrostatic image on the surface of the dielectric receptor. This dielectric receptor is not photosensitive and is generally not re-useable. Once the image pattern has been “written” onto the dielectric receptor in the form of an electrostatic charge pattern of positive or negative polarity, oppositely charged toner particles are applied to the dielectric receptor in order to develop the latent image. An exemplary electrostatic imaging process is described in U.S. Pat. No. 5,176,974. In contrast, electrophotographic imaging processes typically involve the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, cleaning, and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively exposing the photoreceptor to electromagnetic radiation, thereby discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. The electromagnetic radiation, which can also be referred to as “light,” can include infrared radiation, visible light, and ultraviolet radiation, for example.
In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using a developer electrically-biased to a potential having the same polarity as the toner polarity. The toner particles migrate to the photoreceptor and selectively adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor.
In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed. Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Electrophotographic imaging processes can also be distinguished as being either multi-color or monochrome printing processes. Multi-color printing processes are commonly used for printing graphic art or photographic images, while monochrome printing is used primarily for printing text. Some multi-color electrophotographic printing processes use a multi-pass process to apply multiple colors as needed on the photoreceptor to create the composite image that will be transferred to the final image receptor, either by via an intermediate transfer member or directly. One example of such a process is described in U.S. Pat. No. 5,432,591.
A single-pass electrophotographic process for developing multiple color images is also known and can be referred to as a tandem process. A tandem color imaging process is discussed, for example in U.S. Pat. No. 5,916,718 and U.S. Pat. No. 5,420,676. In a tandem process, the photoreceptor accepts color from developer stations that are spaced from each other in such a way that only a single pass of the photoreceptor results in application of all of the desired colors thereon.
Alternatively, electrophotographic imaging processes can be purely monochromatic. In these systems, there is typically only one pass per page because there is no need to overlay colors on the photoreceptor. Monochromatic processes may, however, include multiple passes where necessary to achieve higher image density or a drier image on the final image receptor, for example.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles.
In electrographic printing with dry toners the durability (e.g. erasure and blocking resistance) and archivability of the toned image on a final image receptor such as paper is often of critical importance to the end user. The nature of the final image receptor (e.g. composition, thickness, porosity, surface energy and surface roughness), the nature of the fusing process (e.g. non-contact fusing involving a heat source or contact fusing involving pressure, often in combination with a heat source), and the nature of the toner particles (e.g. developed mass per unit area, particle size and shape, composition and glass transition temperature (Tg) of the toner particles and molecular weight and melt rheology of the polymeric binders used to make the toner particles), may all affect the durability of the final toned image as well as the energy required to heat the fuser assembly to the proper fusing temperature. The proper fusing temperature is operationally defined as the minimum temperature range above the Tg at which the fused toned image develops sufficient adhesion to the final image receptor to resist removal by abrasion or cracking (see, e.g., L. DeMejo, et al., SPIE Hard Copy and Printing Materials, Media, and Process, 1253, 85 (1990); and T. Satoh, et al., Journal of Imaging Science, 35 (6), 373 (1991).). Minimizing the proper fusing temperature is desirable because the time required to heat the fuser assembly to the proper temperature will be reduced, the power consumed to maintain the fuser assembly at the proper temperature will be reduced, and the thermal demands on the fuser roll materials will be reduced if the minimum fusing temperature can be reduced. The art continually searches for improved dry toner compositions that produce high quality, durable images at low fusion temperatures on a final image receptor.